S3FilesStore can use a lot of memory

Hi,

@nramirezuy and me were debugging memory issue with one of the spiders some time ago, and it seems to be caused by ImagesPipeline + S3FilesStore. I haven't confirmed that it was the cause of memory issue, this ticket is based solely on reading the source code.

FilesPipeline reads the whole file to memory and then defers the uploading to thread (via S3FilesStore.persist_file, passing file contents as bytes). So there could be many files loaded to memory at the same time, and as soon as files are downloaded faster than they are are uploaded to s3, memory usage will grow. This is not unlikely IMHO because s3 is not super-fast. For ImagesPipeline it is worse because it uploads not only the image itself, but also the generated thumbnails.

I think S3FilesStore should persist files to temporary location before uploading them to S3 (at least optionally). This would allow streaming files without storing them in memory.